Various official regulations exist prescribing that the engines of automobiles must not exceed certain emission limit values, such as, for example, ECE Regulation R 83 for the European Community, or the Code of Federal Regulations No. 40 for the United States. For the most part, these standards prescribe, apart from limit values for emissions, the manner in which sampling is to be performed by use of systems with variable dilution that are provided for measurement of the emissions.
Systems of the above type are known, for example, under the term “CVS system” (constant volume sampling). In these systems, the exhaust gas is always admixed with such a quantity of air that a constant total volume flow of the air/exhaust gas mixture is generated. The samples, which in these systems are taken in bags, are subsequently be analyzed with respect to their pollutant content. The carbon dioxide, carbon monoxide, carbon hydride and nitrogen oxide contents are, for example, measured. While a particle measurement up to now had to be performed only for diesel engines, this measurement will in the future be obligatory also for Otto engines with direct injection.
An advanced variant of a CVS system for taking exhaust gas samples is described in DE 693 15 463 T2. This system comprises an exhaust gas inlet and an air inlet with an air filter upstream thereof. Via a controlled pump, the two gas flows are sucked into a following mixing zone from which they will proceed, while being mixed as homogeneously as possible, into a dilution tunnel. In the dilution tunnel, at a sufficient distance from the mixing zone, a subsonic Venturi nozzle is arranged which is connected to a channel for taking a sample therefrom, the gas flow through this channel being generated by a second pump. In the further course of the dilution tunnel upstream of the conveying pump, the dilution channel is narrowing again in the form of a subsonic Venturi nozzle. Via these Venturi nozzles, the mass flow velocities are detected which are set to be proportionate to each other. The pressure or temperature of the mixed gas are detected via various further sensors and communicated to a control unit by which the proportionality of the mass flow velocities at the two Venturi nozzles and an identical effective pressure shall be safeguarded.
Such a system, also known as CVS systems, is not, however, suited for use as a system for sampling exhaust gas in Otto and diesel engines because one would have to expect particle and carbon hydride deposits to occur in the region of the conduit system, which would distort a subsequent measurement.
The present state of developments therefore resides in the use of two parallel dilution tunnels with two sampling probes wherein the required conveying speed is generated only via a common conveying pump.
Even though DE 195 05 415 A1 describes an exhaust gas test stand wherein two roller-type test stands are connected to a single CVS system, DE 195 05 415 A1 merely provides switching over the exhaust gas inlet, thus making it impossible to use one branch for diesel engines and one branch for Otto engines because, particle and HC residues would then be expected in the area of the mixing zone.